1. Field of the Invention
The invention relates to scanning channels in wireless networks, and more particularly, to a method for adjusting scanning parameters used for scanning channels in a predetermined wireless band in order to optimize the scanning parameters according to congestion levels of the channels.
2. Description of the Prior Art
Wireless local area networks (WLANs) typically transmit via radio or infrared frequencies to connect data devices. WLANs link portable and wireless computer devices, also called mobile stations or terminals, to a wired LAN via a plurality of fixed access points (APs), also called base stations. Allowing WLAN devices to communicate with the infrastructure network, access points provide for wireless communications within respective cells and are typically spaced throughout a designated networked area. The access points facilitate communications between a networked set of 802.11-compliant devices called a basic service set (BSS), as well as communications with other BSSs and wired devices in or connected to wired infrastructure network systems.
The most common WLAN technologies are described in the Institute of Electrical and Electronics Engineer' s IEEE 802.11 family of industry specifications, which include two physical-layer standards: 802.11b operating at 2.4 GHz and delivering up to 11 Mbps at 250 feet maximum; and 802.11a operating at 5 GHz and delivering up to 54 Mbps at 150 feet maximum. A third standard, 802.11g, provides the speeds of 802.11a at the distances of 802.11b. Other wireless standards are also used that use other wireless bands, each operating at a specific frequency range and corresponding bandwidth, as shown in FIG. 1. FIG. 1 depicts a chart showing various wireless bands defined as industrial, scientific and medical (ISM) radio bands as well as wireless bands defined as Unlicensed National Information Infrastructure (U-NII) radio bands. The chart shown in FIG. 1 is not an exhaustive list of radio bands, and is only used as an example. These ISM and U-NII bands are unlicensed frequency bands used in the 802.11b, 802.11a, 802.11g, and 802.11n specifications. For each band, there are some non-overlapped channels used in the same area with no interference between them, as will be explained below.
Before a WLAN device can communicate with other devices in a given WLAN, it must first locate access points. The IEEE 802.11 medium access control (MAC) protocol defines beacon frames sent at regular intervals, known as beacon intervals, for example, every 100 microseconds, by an access point that allow WLAN devices to monitor for the presence of an access point. Passive and active scanning techniques have been developed for WLAN devices to detect access points, although the 802.11 standard does not mandate particular methods for scanning.
Passive scanning allows the network interface card (NIC) of a WLAN device to find an IEEE 802.11 network by listening for traffic. As defined in 802.11, passive scanning involves a WLAN device listening to each frequency channel for no longer than a maximum duration. In this passive mode, the wireless NIC listens for beacons from neighboring access points, while extracting information about the particular channel. Passive scanning expends time and battery power while listening for a beacon frame that may never occur or may be on an idle channel.
The scanning time used for passive scanning is configured during the initialization stage of the WLAN device driver. To initiate a passive scan, the driver commands the firmware to perform a passive scan with a list of channels. The firmware sequences through the list of channels and sends any received frames to the driver. The driver is able to abort the passive scan when the desired beacon or probe response is received.
Active scanning, in contrast to passive scanning, requires the scanning wireless NIC to transmit requests and receive responses from other 802.11 wireless NICs and access points. Active scanning allows the mobile wireless NIC to interact with another wireless NIC or access point based on probe requests and probe responses.
The active scanning of the IEEE 802.11 MAC uses a set of management frames including probe request frames that are sent by a WLAN device and are followed by probe response frames sent by an available access point. In this way, a WLAN device may scan actively to locate an access point operating on a certain channel frequency and the access point can indicate to the WLAN device what parameter settings it is using.
In an active scan, the WLAN device transmits a probe request frame, and if there is a network on the same channel that matches the service set identity (SSID) in the probe request frame, an access point in that network will respond by sending a probe response frame to the WLAN device. The probe response includes information the WLAN device uses to access a description of the network. The WLAN device processes the beacon frames and any additional probe responses that it may receive.
Once the various responses are processed or it has been determined that no response has been received within a prescribed time, a WLAN device may continue to scan on another radio channel. At the end of the scanning process, the WLAN device has accumulated data about the networks in its vicinity, and the device can determine which network to join.
Please refer to FIG. 2. FIG. 2 illustrates an operating environment in which communication units (CU) 20, 22, 24, 26 attempt to wirelessly communicate with access points (AP) 10, 12, 14. Each access point 10, 12, 14 is deployed on a specific channel, such as channel 1 or 9. To communicate with each other, communication units 20, 22, 24, 26 should first join a basic service set (BSS) or an extended service set (ESS). To associate with a BSS, communication units 20, 22, 24, 26 scan over the channels to find the candidate access point among the available access points 10, 12, 14. If the candidate access point is found in the scanning results, the communication units 20, 22, 24, 26 will issue a request to associate with the candidate access point over a wireless communication channel.
Please refer to FIG. 3. FIG. 3 shows 13 channels used in the 2.4 GHz band, and illustrates the overlapping nature of the channels. As shown in FIG. 3, channels that are close to each other, such as channels 5 and 6, overlap to a large degree. Channels that are farther apart from one another, such as channels 1 and 6, do not overlap to any meaningful extent. Overlapping channels can cause interference with one another, and as a result, many device manufacturers, such as access point manufacturers, prefer to configure their devices so that non-overlapping channels are used for signal transmission and reception in order to minimize conflicts and interference between the channels. Based on the defined channel scheme of 802.11b/g for the North American domain within the 2.4 GHz band, up to three non-overlapping channels can be used. FIG. 3 illustrate that channels 1, 6, and 11 are the typical three non-overlapping channels used, and these channels are emphasized in bold in FIG. 3. Each of the channels in this group of non-overlapping channels operates at a frequency that is spaced far enough apart from frequencies of the other channels in the group that the channels do not interfere with one another. Other groups of non-overlapping channels can be created, such as channels 2, 7, and 12 or channels 3, 8, and 13, but at most only three non-overlapping channels can be used in a single group according to the specification illustrated in FIG. 3 without any modifications.
Although different non-overlapping channel group combinations are possible, many access points and other wireless devices sold use channels 1, 6, and 11. As a result, channels 1, 6, and 11 are often the busiest channels in use. When a wireless communication unit wishes to scan each channel in a wireless band to completely discover available access points, a full scan must be performed. Unfortunately, some access points may not be discovered during the full scan if each channel is scanned for the same period of time. As some channels are more congested than others, access points using the congested channels of the wireless band may not be discovered during the full scan process. If a known access point is not included in the full scan results, another full scan should be started, thereby wasting considerable time and network resources. As a result, a better scanning method is needed to address these problems.